Topic 10 - Electricity and Circuits

Question 1

10.1 - What is the structure of an atom?

Answer 1

Atoms have a nucleus at its center that has positively charged protons and neutral neutrons. These particles have similar mass.
Electrons are found at different distances from the nucleus, in shells. Electrons are smaller and negatively charged equal but opposite to a proton.
There are the same amount of protons as neutrons so the overall atom is uncharged.

Question 2

10.2 - What are the conventions of positive and negative terminals?

Answer 2

A metal wire has many free electrons. When a battery is attached to it, the voltage pushes free electrons around the circuit..
Electrons are negatively charged so they move towards the positive terminal of the battery.
However, the conventional direction of current is used, from the positive terminal to the negative terminal of the battery.

Question 3

10.2 - What are the symbols representing different components?

Answer 3

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Question 4

10.3 - Describe the differences between series and parallel circuits

Answer 4

In series circuits components are connected in a way in which the current can take only one route around the circuit.
In parallel circuits, there are junctions allowing the current to take different routes.
In a series circuit, lamps can’t be switched on and off individually.
In parallel circuits each lamp can be switched separately.

Question 5

10.4 - How is potential difference measured?

Answer 5

It is measured in volts, using a voltmeter, it is always connected in parallel to a to measure the p.d across a component.

Question 6

10.5 - What is potential difference?

Answer 6

When a cell transfers energy to the charge, the charge gives energy to the components of the circuit, giving it potential energy.
Therefore p.d is the energy transferred per unit charge passed.
Therefore 1 volt = 1 joule per coloumb ( current )

Question 7

10.6 - What equation links energy, current and p.d?

Answer 7

energy transferred = charge moved * potential difference.
E = QV

Question 8

10.7 - How is current measured?

Answer 8

It is measured in amps, using an ammeter. This is connected in series to measure the current passing through a component or circuit.

Question 9

10.8 - What is current?

Answer 9

Moving charged particles form a current, charge is measured in coulombs.
One coulomb is the charge passing a point in a circuit when there is a current of 1 amp for 1 second.
Therefore current is the rate of flow of charge.

Question 10

10.9 - What equation links charge, current and time?

Answer 10

Charge = current * time.
Q = It

Question 11

10.10 - How does p.d cause current to flow?

Answer 11

P.d or voltage is needed to push current around a circuit.
Electrons flow when a p.d is applied across a component.
For a current to flow, the circuit must be closed and have a source of p.d like a battery.
The bigger the p.d, the bigger the current.

Question 12

10.11 - What happens to current and p.d at a junction in a circuit?

Answer 12

Current leaving the positive terminal is the same as the current arriving at the negative terminal.
This is because current is conserved.
In a parallel circuit, the current splits at junctions, but the total amount entered is the same as the amount leaving.

In a parallel circuit, the p.d across each branch is the same, when there is more than one component in a branch, the p.d is shared.
Although the p.d in the circuit is conserved as the p.d in = the p.d out.

Question 13

10.12 - How does changing resistance change the current?

Answer 13

A large electrical resistance may lead to a lower p.d and this leads to lower current.
Resistance is measured in ohms (Ω).

Question 14

10.13/15 - What equation links voltage, current and resistance?

Answer 14

Voltage = current * resistance
V = IR

Question 15

10.14 - How does resistance change with more resistors in series and parallel circuits?

Answer 15

In series circuits, the total resistance of the circuit increases as the resistors are connected.
This is because the pathway becomes harder for current to flow.
P.d from a cell is shared between resistors, but it may not be shared equally, resistors with a higher resistance have a greater p.d across it.

In parallel circuits, when resistors are connected, the resistance decreases.
This is because there are more paths for current compared to a series circuit.

Question 16

10.16 - Explain the design of series circuits for testing resistance

Answer 16

A variable resistor is used to change the current in the circuit.
Measurements of current and voltage are taken to calculate the resistance.

Question 17

10.17 - Core Practical : Investigate the relationship between p.d, current and resistance in a circuit

Answer 17

Set up a circuit with an ammeter, voltmeter, power pack and resistor.
Set the power pack to the lowest voltage and switch on, note the readings on the voltmeter and ammeter.
Repeat for 5 different voltage settings, up to 6V.
Replace the resistor in the circuit with two filament lamps and repeat.

Question 18

10.17 - Core Practical : Investigate how voltage and current change in a series and parallel circuit.

Answer 18

Set up a series circuit with 2 lamps, a power pack, an ammeter and 3 voltmeters.
Set the power pack to the lowest voltage and switch on, note the readings on the voltmeter and ammeter.
Repeat for 5 different voltage settings, up to 6V.
Set up a parallel circuit with 2 lamps, a power pack, 3 ammeter and 3 voltmeters.
Set the power pack to the lowest voltage and switch on, note the readings on the voltmeter and ammeter.
Repeat for 5 different voltage settings, up to 6V.

Question 19

10.18 - Explain how current varies with p.d in components

Answer 19

When p.d changes across a fixed resistor, the current changes by the same amount. The two variables are in direct proportion.
The graph is a linear line through the origin.

P.d across a filament lamp causes a current to flow through it. This causes the filament to heat up.
Greater the p.d, more current flows and hotter the filament gets. But as it heats up, the filament’s resistance increases, this decreases the p.d.
As the p.d decreases, it won’t be in direct proportion to the current.

A diode has low resistance if p.d is in one direction, however if in the opposite direction, the resistance is very high.
This means current can only flow in one direction.

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Question 20

10.19 - How does the resistance of an LDR vary?

Answer 20

A light-dependent resistor ( LDR ) has a high resistance in the dark but as the resistance decreases, light intensity increases.

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Question 21

10.20 - How does the resistance of a thermistor vary?

Answer 21

Thermistors have high resistances at low temperature but as the the temperature increases the resistance decreases.

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Question 22

10.21 - Explain the design of series circuits for testing resistance in different components

Answer 22

In a series circuit, the change in resistance of a lamp when p.d across the lamp is changed can be measured by using:
A variable resistor to change the p.d from the battery.
A voltmeter to measure p.d of the lamp.
And an ammeter to measure the current.

Question 23

10.22/23/26 - What are the advantages and disadvantages of the heating effect of a current?

Answer 23

All circuits have some resistance, heating them up when there is a current.
When a current passes through a resistor, it warms up due to energy transfers from the electrical work done against the resistance.
This effect is useful in heaters and kettles, although it isn’t useful in computers or plugs.
This is because the useful energy is transferred from the circuit and is dissipated so the surroundings gain thermal energy.

Question 24

10.24 - Explain the heating effect of a current at an atomic level

Answer 24

As the electrons of the resistor move through the lattice of vibrating ions, they collide with them.
More collisions made with ions, harder it is for them to pass through, so higher the electrical resistance.
When the electrons collide with the ions they transfer energy to them, heating the resistor up.

Question 25

10.25 - Explain ways of reducing unwanted energy transfer through wires

Answer 25

Resistance in circuits can be reduced by using wires made of metals that are less resistant like copper.
Thicker wires have more electrons to carry the current so have lower resistance.
Resistance can also be decreased by cooling down metals so the lattice ions aren’t vibrating as much.

Question 26

10.27 - What equation links energy, current, voltage and time?

Answer 26

Energy = current * voltage * time
E = IVt

Question 27

10.28 - What is power?

Answer 27

Power is the energy transferred per second.
Power is measured in watts.
1W = 1 joule per second

Question 28

10.29 - What equation links power, energy and time?

Answer 28

Power = energy/time
P = E/t

Question 29

10.30 - How is power transfer in a circuit related to its p.d and current?

Answer 29

Power transfer is proportional to p.d and current.
Therefore the equation: Power = current * voltage
P =IV

Question 30

10.31 - What equation links power, current and resistance?

Answer 30

Power = current squared * resistance
P = I^2 * R

This is derived from P = IV and V = IR.
Plugging V into the first equation gives P = I^2 * R

Question 31

10.32/42 - How is energy transferred from electrical cells to motors in different appliances?

Answer 31

A battery operated fan has energy stored in its battery, this is transferred by electricity to the motor, where it is transferred to a store of KE in the fan.
Some energy will also heat the wires, the motor and surroundings.
This energy will be dissipated and increases the thermal energy of the surroundings.

Gloves that contain wires with high resistance has energy stored in its battery.
This is transferred by electricity to the high resistance wires, where it is transferred to a thermal energy store in the wire.
This energy is then transferred to the hands and then the thermal energy of the surroundings.

Question 32

10.33/34/35/36 - Describe the differences between direct and alternating current

Answer 32

Direct current is when the movement of charge stays the same, this is supplied by batteries and cells.
Mains electricity is produced using generators that rotate, this changes the direction of current, this is alternating current.

In AC current, the voltage also changes, from peak to 0 and then decreasing to peak in opposite direction and decreases back to zero, this cycle carries on.
In the UK, 50 of these cycles happen per second, this means the frequency of the domestic power supply is 50 Hz.
The voltage of the supply changes in AC but the output voltage of AC and DC are 230V.

Question 33

10.37 - Explain the difference between live and neutral mains input wires

Answer 33

Live wires connect appliances with power station and its voltage is 230V.
Neutral wires bring current from the power station and its voltage is 0V.

Question 34

10.38 - Explain the function of an earth wire, fuses and circuit breakers for safety

Answer 34

Metal parts of the appliance connected to the Earth for safety, the voltage is usually 0V.
This is because in an electric shock, a current flows through the body to the earth. Instead of this happening, current goes through the earth wire to the earth.
A fuse is a thin wire that passes current , if the current exceeds a certain value and gets too hot, the fuse melts, this breaks the circuit.

Circuit breakers detect a change in the current and switch off the mains supply.
This is useful, as once the fault is fixed, the mains can be switched on again, whereas a fuse is replaced.
It can also work quickly, saving lives compared to a fuse which takes time to melt.

Question 35

10.39 - Explain why fuses should be connected to the live wire

Answer 35

Because if the live wire touches a metal part, it makes a low resistance circuit between it and the earth, this causes large amounts of current to flow to the earth.
This heats up the the wire and causes the current to melt the fuse and cut off the mains electricity supply.

Question 36

10.40 - What is the p.d of the live, neutral and earth wire?

Answer 36

Live - 230V to -230V
Neutral - 0V
Earth - 0V

Question 37

10.41 - Explain the dangers of providing connection between the live wire and earth

Answer 37

If a faulty wire draws too much current it causes overheating of the wires and appliance causing fires.
The metal parts of the appliance can have high voltage which can cause electric shocks.